High Thermal Stability by Doping of Oxide Capping Layer for Spin Torque Transfer (STT) Magnetic Random Access memory (MRAM) Applications

ABSTRACT

A magnetic tunnel junction (MTJ) is disclosed wherein a free layer (FL) interfaces with a metal oxide (Mox) layer and a tunnel barrier layer to produce interfacial perpendicular magnetic anisotropy (PMA). The Mox layer has a non-stoichiometric oxidation state to minimize parasitic resistance, and comprises a dopant to fill vacant lattice sites thereby blocking oxygen diffusion through the Mox layer to preserve interfacial PMA and high thermal stability at process temperatures up to 400° C. Various methods of forming the doped Mox layer include deposition of the M layer in a reactive environment of O 2  and dopant species in gas form, exposing a metal oxide layer to dopant species in gas form, and ion implanting the dopant. In another embodiment, where the dopant is N, a metal nitride layer is formed on a metal oxide layer, and then an anneal step drives nitrogen into vacant sites in the metal oxide lattice.

PRIORITY DATA

The present application is a divisional application and claims thebenefit of U.S. patent application Ser. No. 15/728,818 filed Oct. 20,2017, herein incorporated by reference in its entirety.

RELATED PATENT APPLICATION

This application is related to the following: Docket # HT17-014, Ser.No. 15/841,479, filing date Dec. 14, 2017; which is assigned to a commonassignee and herein incorporated by reference in its entirety.

TECHNICAL FIELD

The present disclosure relates to a magnetic tunnel junction (MTJ)comprised of a free layer that interfaces with a tunnel barrier layerand a Hk enhancing layer that is a metal oxide, and in particular toreducing the Hk enhancing layer resistance and minimizing diffusion ofoxygen from the metal oxide/free layer interface to provide highperpendicular magnetic anisotropy (PMA) in the free layer that enablesthermal stability in the memory device for process temperatures up to400° C.

BACKGROUND

STT-MRAM technology for writing of memory bits was described by C.Slonczewski in “Current driven excitation of magnetic multilayers”, J.Magn. Magn. Mater. V 159, L1-L7 (1996), and is highly competitive withexisting semiconductor memory technologies such as SRAM, DRAM, andflash. STT-MRAM has a MTJ cell based on a tunneling magnetoresistance(TMR) effect wherein a MTJ stack of layers has a configuration in whichtwo ferromagnetic layers are separated by a thin insulating tunnelbarrier layer. One of the ferromagnetic layers called the pinned layerhas a magnetic moment that is fixed in a perpendicular-to-planedirection. The second ferromagnetic layer (free layer) has amagnetization direction that is free to rotate between a directionparallel to that of the pinned layer (P state) and an antiparalleldirection (AP state). The difference in resistance between the P state(Rp) and AP state (Rap) is characterized by the equation (Rap−Rp)/Rpthat is also known as DRR. It is important for MTJ devices to have alarge DRR value, preferably higher than 1, as DRR is directly related tothe read margin for the memory bit, or how easy it is to differentiatebetween the P state and AP state (0 or 1 bits).

State of the art STT-MRAM structures preferably have a free layer withhigh PMA to allow data retention at small device sizes. For functionalMRAM and STT-MRAM products, the free layer (information storage layer)must have a high enough energy barrier (E_(b)) to resist switching dueto thermal and magnetic environmental fluctuations. The valueΔ=kV/k_(B)T is a measure of the thermal stability of the magneticelement where kV is also known as E_(b) between the two magnetic states(P and AP), k_(B) is the Boltzmann constant, and T is the temperature.This energy barrier to random switching is related to the strength ofthe perpendicular magnetic anisotropy (PMA) of the free layer. Onepractical way to obtain strong PMA is through interfacial PMA at aninterface between an iron rich free layer and a MgO tunnel barrierlayer. This combination enables good lattice matching as well as thepossibility to use MgO as a spin filtering element thereby providing aread signal for the device. Since the writing current density andvoltage across the device is significant, this spin filtering elementmust have high structural quality to sustain billions of write cyclesduring the lifetime of the memory device.

Recent free layer designs have incorporated a second free layer/metaloxide interface on an opposite side of the free layer with respect tothe tunnel barrier to achieve even higher PMA due to an additionalinterfacial PMA contribution. Therefore, total PMA in the free layer isenhanced with a MgO/CoFeB free layer/MgO stack, for example, that alsoincreases E_(b) and thermal stability. The spin filtering capability ofthe second metal oxide layer that is also referred to as a Hk enhancinglayer is typically not used. Because the second metal oxide layercontributes to the total resistance of the device without affecting theread signal, it is engineered to have as low resistance as possible.

Equation (1) shows the effect of the second metal oxide (mox) layerresistance contribution to total MTJ resistance while Equation (2)indicates a negative impact (reduction) for DRR.

$\begin{matrix}{{{DRR} = {\frac{R_{AP} - R_{P}}{R_{P}}\mspace{14mu} {where}}}{R_{AP} = {R_{AP}^{barrier} + {R_{AP}^{mox}\mspace{14mu} {and}}}}{R_{P} = {R_{P}^{barrier} + R_{P}^{mox}}}{{{Since}\mspace{14mu} R_{AP}^{mox}} = R_{P}^{mox}}} & {{Eq}.\mspace{11mu} (1)} \\{{DRR} = {\frac{R_{AP}^{barrier} + R_{AP}^{mox} - \left( {R_{P}^{barrier} + R_{P}^{mox}} \right)}{R_{P}^{barrier} + R_{P}^{mox}} = \frac{R_{AP}^{barrier} - R_{P}^{barrier}}{R_{P}^{barrier} + R_{P}^{mox}}}} & {{Eq}.\mspace{11mu} (2)}\end{matrix}$

In summary, the series resistance caused by the second metal oxide layer(R_(AP) ^(mox) and R_(p) ^(mox)) will cause a reduction in DRR,effectively reducing the STT-MRAM (or MRAM) bit reading margin, as wellas increasing the bit's writing voltage by adding a series resistance.Since a MgO Hk enhancing layer or the like is required to achieve strongPMA for enhanced thermal stability, an improved second metal oxide layerstructure is needed such that high interfacial PMA is maintained at thefree layer interface while significantly reducing the series resistancecontribution from the second metal oxide layer.

Generally, low resistance in a Hk enhancing layer is achieved through alower (non-stoichiometric) oxidation state, or thinning a fully oxidizedlayer. However, the latter is difficult to accomplish without oxidizinga portion of the free layer. Unfortunately, with regard to anon-stoichiometric oxidation state, oxygen vacancies in the metal oxidelayer decrease the thermal stability for the device, and allow forincreased mobility of oxygen within the layer, and greater diffusion ofmetal atoms such as Ta from adjacent layers. Since STT-MRAM devices arelikely to be integrated in standard Complementary Metal OxideSemiconductor (CMOS) processes comprising 400° C. anneal cycles totalingup to 5 hours, this high temperature combined with highly mobile oxygenin a MgO Hk enhancing layer often results in a loss of interfacial PMAat the free layer/Hk enhancing layer interface, and degraded free layerproperties. Thus, an improved Hk enhancing layer design must alsoprovide a means of minimizing oxygen diffusion and metal diffusionthrough the layer in order to preserve high PMA in the free layer anddevice thermal stability up to 400° C.

SUMMARY

One objective of the present disclosure is to provide a MTJ having afree layer that interfaces with a tunnel barrier layer and a Hkenhancing layer wherein the resistance contribution of the Hk enhancinglayer is substantially reduced compared with a fully oxidized layerwhile interfacial PMA is maintained in the free layer to enable MTJthermal stability up to 400° C. process temperatures.

A second objective is to provide a Hk enhancing layer according to thefirst objective that also substantially reduces diffusion of oxygen andother species through the Hk enhancing layer thereby preserving freelayer magnetic properties.

A third objective is to provide a method of forming the Hk enhancinglayer that satisfies the first two objectives.

According to the present disclosure, there is a plurality of embodimentswhereby the aforementioned objectives are achieved. All embodimentsrelate to a MTJ structure comprising a free layer that is formed betweena tunnel barrier layer and a Hk enhancing layer. Furthermore, allembodiments are based on the key feature of incorporating a dopant inthe vacant lattice sites within a Hk enhancing layer having anon-stoichiometric oxidation state thereby preventing or substantiallyreducing the tendency of oxygen and other species to diffuse through themetal oxide lattice structure by a so-called “hopping” mechanism. Thus,the Hk enhancing layer is preferably a metal oxide layer with asubstantial number of under oxidized metal atoms such that there aremetal (conductive) channels between top and bottom surfaces of the Hkenhancing layer to lower the resistance therein. In other words, themetal oxide lattice has a plurality of non-oxygen containing sites thatwould be occupied by oxygen anions in a fully oxidized or stoichiometricoxidation state. Instead, the non-oxygen containing sites are occupiedwith a dopant that is one of N, S, Se, P, C, Te, As, Sb, and Bi. Thus,the dopant will create conducting states in the band gap of a MgO Hkenhancing layer, for example, through hole generation while providing anadditional advantage of blocking oxygen diffusion hopping throughotherwise vacant sites in under oxidized metal oxide layers found in theprior art.

According to various embodiments of the doped Hk enhancing layerdescribed herein, oxygen in the under oxidized Hk enhancing layer doesnot diffuse away from the interface with the free layer, and interfacialPMA is maintained. Moreover, species from adjacent layers such as Taatoms from a cap layer are less likely to diffuse through the Hkenhancing layer and degrade free layer magnetic properties.

According to a first embodiment, the dopant is formed within the Hkenhancing layer during formation of said layer. The metal oxide layermay be formed by first depositing a metal layer such as Mg on the freelayer. Then, the metal layer is subjected to a reactive gas environmentcomprised of flowing oxygen and the dopant in gas form over the metallayer to yield the doped Hk enhancing layer. In other embodiments, ametal oxide such as MgO is sputter deposited from a MgO target in thepresence of a dopant in gas form, or a doped MgO target is sputterdeposited on the free layer. In an alternative embodiment, a freshlyformed MgO layer is prepared by oxidation of a Mg layer or by sputterdeposition of MgO, and then the metal oxide is exposed to the dopant ina reactive gas environment. The present disclosure also encompasses amethod of forming the doped Hk enhancing layer by implanting the dopantinto a metal oxide layer.

According to a second embodiment, the dopant diffuses into the metaloxide Hk enhancing layer during an annealing step subsequent to the Hkenhancing layer deposition. For example, a dopant layer such as MgN orMgON comprised of loosely bound nitrogen may be deposited on a MgO Hkenhancing layer to form a stack of two distinct layers. An anneal stepmay be performed after the entire MTJ stack of layers is formed suchthat a certain amount of the loosely bound nitrogen diffuses into the Hkenhancing layer. In another embodiment, the dopant may be implanted intoan upper portion of the Hk enhancing layer, and then further distributedthrough said layer during a subsequent annealing step.

The present disclosure encompasses a MTJ with a bottom spin valveconfiguration or a top spin valve configuration. In the latter, a seedlayer, doped Hk enhancing layer, free layer, tunnel barrier layer,pinned layer, and cap layer are sequentially formed on a substrate thatmay be a bottom electrode. In the former, a seed layer, pinned layer,tunnel barrier layer, free layer, doped Hk enhancing layer, and caplayer are sequentially formed on the substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view showing a magnetic tunnel junction(MTJ) wherein a free layer is formed between a tunnel barrier layer anda metal oxide (Hk enhancing) layer according the prior art.

FIG. 2 is an enlarged view of the Hk enhancing layer in FIG. 1 wherein aplurality of lattice sites in the under oxidized metal oxide structureare vacant.

FIGS. 3a-3c are diagrams showing various steps of a hopping mechanismwhereby an oxygen anion diffuses between metal cations to a vacant sitein a metal oxide lattice.

FIG. 4 is a cross-sectional view of a MTJ wherein a free layer is formedbetween a tunnel barrier layer and an under oxidized metal oxide layerhaving a plurality of dopant atoms in the metal oxide structureaccording to an embodiment of the present disclosure.

FIG. 5 is an enlarged view of the doped metal oxide layer in FIG. 4where dopant (D) atoms fill essentially all sites in a metal oxidelattice structure that are not occupied by metal (M) or oxygen (O)atoms.

FIG. 6 is a cross-sectional view that depicts a method of forming thedoped metal oxide layer in FIG. 4 by simultaneously depositing thedopant species, metal, and oxygen atoms on a top surface of the freelayer.

FIG. 7 is a cross-sectional view that depicts a method of forming thedoped metal oxide layer in FIG. 4 by exposing a metal layer to areactive gas environment containing oxygen and a dopant speciesaccording to an embodiment of the present disclosure.

FIG. 8 is a cross-sectional view that depicts a method of forming thedoped metal oxide layer in FIG. 4 by exposing a metal oxide layer to areactive gas environment containing a dopant species according to anembodiment of the present disclosure.

FIG. 9 is a cross-sectional view showing an intermediate MTJ stack oflayers comprising a dopant layer on a metal oxide layer that issubsequently converted to a doped metal oxide layer by performing ananneal step.

FIG. 10 is a cross-sectional view showing a partially formed MTJ cellhaving a bottom spin valve configuration shown in FIG. 4 after aphotoresist pattern has been etch transferred through an uppermost caplayer.

FIG. 11 is a cross-sectional view depicting the MTJ cell in FIG. 10following a second etch step to complete the MTJ cell, deposition of anencapsulation layer, and a planarization step according to an embodimentdescribed herein.

FIG. 12 is a top-down view showing a plurality of MTJ cells formed inrows and columns within in a memory array wherein each MTJ cell issurrounded by an encapsulation layer.

FIG. 13 is a cross-sectional view showing a MTJ stack of layers having atop spin valve configuration wherein a free layer is formed between atunnel barrier layer and an under oxidized metal oxide layer having aplurality of dopant atoms therein according to another embodiment of thepresent disclosure.

FIG. 14 is a cross-sectional view depicting the MTJ cell formed afterpatterning the MTJ stack in FIG. 11, depositing an encapsulation layer,and performing a planarization step according to an embodiment of thepresent disclosure.

DETAILED DESCRIPTION

The present disclosure relates to minimizing the resistance contributionof a Hk enhancing layer, and reducing the diffusion of oxygen and otherspecies through said layer in MTJ cells having a tunnel barrierlayer/free layer/Hk enhancing layer configuration thereby enabling ahigh magnetoresistive ratio and sufficient PMA in the free layer for MTJthermal stability up to 400° C. The MTJ may be formed in a MRAM,STT-MRAM, magnetic sensor, biosensor, spin torque oscillator, or inother spintronic devices known in the art. Only one MTJ cell is depictedto simplify the drawings, but typically the memory devices describedherein contain millions of MTJs that are arrayed in rows and columns ona substrate. The terms “non-stoichiometric” and “under oxidized” areused interchangeably when referring to an oxidation state of a Hkenhancing layer wherein metal atoms in a metal oxide layer are not fullyoxidized. An interface that produces interfacial PMA is defined as aboundary region comprised of a free layer surface and an adjoiningsurface of a metal oxide layer that may be a tunnel barrier layer or Hkenhancing layer. The term “Hk enhancing” refers to a metal oxide layerthat increases PMA in the free layer as a result of the metal oxideforming an interface with the free layer.

Referring to FIG. 1, the inventors have previously fabricated a MTJ cell1 with a patterned stack of layers consisting of seed layer 11, pinnedlayer 12, tunnel barrier 13, free layer 14, Hk enhancing layer 15, andcap layer 16 that are sequentially formed on top surface 10 t ofsubstrate 10. The substrate may comprise a bottom electrode formed on asubstructure comprised of transistors, and a conductive layer with aplurality of bit lines (not shown) that are electrically connected tothe bottom electrode layer through vias and the like.

The optional seed layer 11 is comprised of one or more of NiCr, Ta, Ru,Ti, TaN, Cu, Mg, or other materials typically employed to promote asmooth and uniform grain structure in overlying layers.

Pinned layer 12 may have a synthetic anti-parallel (SyAP) configurationrepresented by AP2/Ru/AP1 where an anti-ferromagnetic coupling layermade of Ru, Rh, or 1r, for example, is sandwiched between an AP2magnetic layer and an AP1 magnetic layer (not shown). The AP2 layer,which is also referred to as the outer pinned layer is formed on theseed layer while AP1 is the inner pinned layer and typically contactsthe tunnel barrier. AP1 and AP2 layers may be comprised of CoFe, CoFeB,Co, or a combination thereof. In other embodiments, the pinned layer maybe a laminated stack with inherent PMA such as (Co/Ni)_(n),(CoFe/Ni)_(n), (Co/NiFe)_(n), (Co/Pt)_(n), (Co/Pd)_(n), or the likewhere n is the lamination number. Furthermore, a transitional layer suchas CoFeB or Co may be inserted between the uppermost layer in thelaminated stack and the tunnel barrier layer 13.

Tunnel barrier layer 13 is preferably a metal oxide that is one of MgO,TiOx, AlTiO, MgZnO, Al₂O₃, ZnO, ZrOx, HfOx, or MgTaO, or a lamination ofone or more of the aforementioned metal oxides. More preferably, MgO isselected as the tunnel barrier layer because it provides the highestmagnetoresistive ratio (DRR).

Free layer 14 may be Fe, CoFe, or an alloy thereof with one or both of Band Ni, or a multilayer stack comprising a combination of theaforementioned compositions wherein the Fe content is greater than 50atomic % (iron rich) of the total content of magneticelements/constituents. For example, in a Co_((100-x))Fe_(x)B free layer,x is greater than 50 atomic %. In some embodiments, the free layer has aSyAP configuration such as FL1/Ru/FL2 where FL1 and FL2 are two ironrich magnetic layers that are antiferromagnetically coupled through a Rulayer. In yet another embodiment, the free layer is comprised of a highKu material having inherent PMA such as a Heusler alloy that is Ni₂MnZ,Pd₂MnZ, Co₂MnZ, Fe₂MnZ, Co₂FeZ, Mn₃Ge, or Mn₂Ga where Z is one of Si,Ge, AI, Ga, In, Sn, and Sb. Moreover, the free layer may be an orderedL1 ₀ or L1 ₁ material with a composition that is one of MnAI, MnGa, oran alloy RT wherein R is Rh, Pd, Pt, Ir, or an alloy thereof, and T isFe, Co, Ni, or alloy thereof, or is a rare earth alloy with a TbFeCo,GdCoFe, FeNdB, or SmCo composition.

Hk enhancing layer 15 is typically a metal oxide layer such as MgO thathas a non-stoichiometric oxidation state so that the resistancecontribution R** found in the denominator of equation (2) is minimizedthereby reducing the adverse effect on DRR. In related patentapplication HT17-014, we disclosed additional schemes for reducing theresistance contribution from a Hk enhancing layer that generally involvethe formation of conductive pathways through the metal oxide layer.

Cap layer 16 is non-magnetic and serves as a hard mask for etchprocesses that determine the shape of the MTJ cell. The cap layer may becomprised of one or more conductive metals or alloys including but notlimited to Ta, Ru, TaN, Ti, TiN, W, and MnPt. Furthermore, the cap layermay comprise an electrically conductive oxide such as RuOx, ReOx, IrOx,MnOx, MoOx, TiOx, or FeOx.

As mentioned earlier, an under oxidized Hk enhancing layer 15 shown inFIG. 2 has vacant sites V in the metal oxide lattice structure that iscomprised of metal (M) cations and oxygen (O) anions. In the example, a(001) type plane of MgO in a non-stoichiometric oxidation state isdepicted wherein there is a plurality of vacant sites between metalcations. As a result, oxygen is able to diffuse away from the freelayer/Hk enhancing layer interface (not shown) through a pathwaycomprised of a plurality of vacant sites in a so-called “hopping”mechanism. Likewise, species from other layers such as Ta atoms from thecap layer 16 are able to diffuse through the vacant sites into the freelayer to cause an undesirable decrease in thermal stability and lowerDRR.

Referring to FIGS. 3a-3c , the hopping mechanism is illustrated where anoxygen anion O is positioned to the left of a vacant site V in a metaloxide lattice at a starting point in FIG. 3a . During step s1, there issufficient energy provided by one or more sources such as heat during ananneal step to drive diffusion of the larger oxygen anion between twosmaller M cations to reach an intermediate (higher energy) state shownin FIG. 3b . Note that the relative size of the metal cations and oxygenanions is not necessarily drawn to scale, and the actual space betweenadjacent rows of metal cations may be less than illustrated. Thereafter,in step s2, the oxygen anion reaches a more favorable energy state at apoint shown in FIG. 3c by occupying a former vacant site while a newvacant site is formed to the left of the O anion. It should beunderstood that oxygen anion diffusion between vacant sites may occur inan upward or downward direction (not shown) in addition to a sidewaysmotion through the metal oxide lattice.

Now we have found a Hk enhancing layer design to not only lower theparasitic resistance therein, but also substantially reduce diffusion ofoxygen and other species through the metal oxide lattice in an underoxidized Hk enhancing layer. All embodiments described herein involve aMTJ stack of layers wherein a free layer 14 is sandwiched between atunnel barrier layer 13 and a doped Hk enhancing layer 17 to provide aDRR above 1, and thermal stability up to 400° C. during CMOS processes.The doped Hk enhancing layer contacts the free layer top surface inbottom spin valve configurations, and adjoins the free layer bottomsurface in top spin valve configurations as explained in the followingembodiments.

According to one embodiment of the present disclosure shown in FIG. 4, aMTJ stack of layers 2 having a bottom spin valve configuration retainsall of the layers from FIG. 1 except Hk enhancing layer 15 is replacedwith doped Hk enhancing layer 17. Thus, PMA in free layer 14 is enhancedthrough two interfaces with a metal oxide layer including interface 40with tunnel barrier layer 13, and interface 41 with doped Hk enhancinglayer 17.

Referring to FIG. 5, a key feature of all embodiments described hereinis to provide an under oxidized Hk enhancing layer 17 that comprises adopant to fill essentially all lattice sites in a metal oxide (Mox)layer that are not occupied by metal M cations or oxygen O anions. Here,a plurality of dopant anions (D) is provided in lattice sites betweenmetal cations where M is one or more of Mg, Si, Ti, Ba, Ca, La, Mn, V,Al, or Hf. Note that there are substantially more oxygen anions than Danions, which fill the lattice sites between M cations. The presentdisclosure anticipates the content of dopant in the Hk enhancing layermay be from around 100 ppm up to 20 atomic %. In particular, the dopantmay be one or more of N, S, Se, P, and C. However, Te, As, Sb, or Bi mayalso be selected as a dopant. Dopant anions effectively createconducting states in the band gap of MgO or other metal oxides throughhole generation. Moreover, the D anions block diffusion of oxygenthrough the lattice by filling vacancies that are necessary for oxygendiffusion to occur through a hopping mechanism.

The present disclosure also encompasses a method of incorporating one ormore of the aforementioned dopants in the under oxidized Hk enhancinglayer 17. According to one embodiment depicted in FIG. 6, the dopedmetal oxide layer is formed in a reactive gas environment generated by achemical vapor deposition (CVD), physical vapor deposition (PVD), or aplasma enhanced CVD (PECVD) method wherein dopant species D, metalspecies M, and oxygen species O are simultaneously deposited on a topsurface 14 t of free layer 14. Oxygen and dopant species may be in gasform when generated with the metal species in a reaction chamber. In oneembodiment, the source of the M and O species is a metal oxide (MO)target that is sputtered in the presence of the D species in gas form.In another embodiment, all three species are generated in a reactionchamber by sputtering a MOD target such as MgON, for example. It shouldbe understood that the term “species” may refer to a neutral state suchas O₂, to an ionic state (cation or anion), or to a radical depending onthe temperature and type of power applied to the reaction chamber, andon the source of the dopant species.

In FIG. 7, an alternative embodiment for forming a doped Hk enhancinglayer is illustrated. In particular, a metal film 17 m such as Mg or oneof the other M metals or alloys is first deposited on free layer 14.Then, the metal film is exposed to a reactive gas environment containingoxygen species O and dopant species D in gas form. As a result, the Oand D species diffuse into the metal film and react with M to yielddoped Hk enhancing layer 17 with the lattice structure shown in FIG. 5.

In another embodiment depicted in FIG. 8, an under oxidized Mox layer 17ox with exposed top surface 17 t is formed on the free layer 14. The Moxlayer may be formed by sputtering a Mox target, or by first depositing aM layer and then performing a conventional oxidation that may be anatural oxidation (NOX) comprising a flow of O₂ over the M layer, or aradical oxidation (ROX) where oxygen radicals react with the M layer.Thereafter, dopant species D are provided in a reactive gas environmentthat may be a PVD, CVD, or PECVD process, for example. The dopantspecies diffuse into the Mox layer to form the doped Hk enhancing layer17 in FIG. 4. Alternatively, dopant species may be implanted in the Moxfilm shown in FIG. 8 by employing a conventional ion implantation schemewhereby species of a dopant material are accelerated in an ion beam thatis directed at top surface 17 t.

In some embodiments, the dopant has a substantially uniform distributionthrough Hk enhancing layer 17. However, the present disclosure alsoanticipates a non-uniform distribution of the dopant in the resultingmetal oxide lattice. For example, a higher concentration of dopant maybe formed in an upper portion of the Hk enhancing layer while a lowerdopant concentration is in a lower portion thereof proximate to the freelayer interface.

According to a second embodiment shown in FIG. 9, the dopant is diffusedinto an under oxidized metal oxide layer with an anneal step after theMox layer is formed. Moreover, one or more anneal steps may be employedto more evenly distribute a dopant in the Hk enhancing layer 17. Theexemplary embodiment comprises depositing a dopant layer 17 d on the Moxlayer 17 ox. For example, if the dopant is N, then the dopant layer maybe a metal nitride or metal oxynitride including but not limited toSi₃N₄, MgN, and MgON. Preferably, the metal in the metal nitride ormetal oxynitride layer is the same metal as in the underlying Mox layer.It is believed that loosely bound nitrogen in the nitride or oxynitridelayer is driven into the underlying Mox layer during one or more annealsteps thereby forming a doped and under oxidized Hk enhancing layer 17shown in FIG. 4. In some cases, a substantial portion of the looselybound nitrogen diffuses into the Hk enhancing layer following one ormore anneal steps. The anneal steps may include an anneal step (1) afterall MTJ layers are formed but before patterning to form a MTJ cell, (2)after MTJ cell formation but before a subsequent encapsulation process,(3) during encapsulation, and (4) after encapsulation layer deposition.

In another embodiment similar to the method shown in FIG. 7, the dopantD may be implanted into an upper portion of the Mox layer 17 ox to avoiddriving the dopant all the way through the Mox layer into the free layer14. Then, one or more anneal steps are subsequently performed duringdevice fabrication to diffuse the dopant deeper into the Mox layer. Asmentioned previously, the dopant may be unevenly distributed throughoutthe resulting doped Hk enhancing layer 17 depicted in FIG. 4 such thatthere is a higher concentration in an upper portion of said layer and alower concentration or zero concentration of dopant in a lower portionthereof that is proximate to the free layer. Preferably, theconcentration of dopant proximate to the interface with the free layeris minimized to prevent diffusion into the free layer where the dopantcould alloy with the free layer and lower DRR.

After the doped Hk enhancing layer 17 is formed according to one of theaforementioned embodiments, the cap layer 16 is deposited thereon tocomplete the MTJ stack of layers shown in FIG. 4.

Referring to FIG. 10, a first sequence of steps is shown for patterningthe MTJ stack in FIG. 4. First, a bottom antireflective (BARC) ordielectric antireflective (DARC) layer 45 and a photoresist layer aresequentially formed on cap layer 16. The photoresist layer ispatternwise exposed and developed by a conventional photolithographyprocess to form a pattern comprised of a plurality of photoresistislands 50 each having width w and sidewall 50 s. The photoresistislands are in an array (not shown) and each have a top-down shape thatwill be essentially duplicated in the underlying MTJ cells to be formedin subsequent steps. A first etch step that may be a reactive ion etch(RIE) based on an oxygen and fluorocarbon etchant, for example, isemployed to transfer the photoresist island pattern through theBARC/DARC layer 45 and through cap layer 16 and stopping on top surface17 t of the doped Hk enhancing layer. As a result, cap layer sidewall 16s is preferably coplanar with photoresist sidewall 50 s and BARC/DARCsidewall 45 s.

Referring to FIG. 11, a second RIE or an ion beam etch (IBE) is used totransfer the pattern in the cap layer through underlying layers in theMTJ stack. According to one embodiment, the second RIE or IBE iscomprised of plasma or ions of an oxidant such as methanol, and plasmaor ions of Ar or another noble gas and generates sidewall 2 s on MTJcell 2. However, the present disclosure is not limited to a particularRIE or IBE chemistry and anticipates that other types of etchants may beemployed to form sidewall 2 s.

After, the second RIE or IBE stops on the substrate 10, an encapsulationlayer 20 is deposited on substrate top surface 10 t to fill the gapsbetween MTJ 2 and adjacent MTJs (not shown). The encapsulation layer isa dielectric material and may include a plurality of sub-layers asappreciated by those skilled in the art. Then, a chemical mechanicalpolish (CMP) step or another planarization method is performed to form atop surface 20 t on the encapsulation layer that is coplanar with topsurface 16 t of the cap layer. Any photoresist or BARC/DARC materialremaining after the second RIE or IBE step is removed by the CMP step.

A top-down view of the MTJ structure after the planarization step isshown in FIG. 12. Each MTJ cell 2 is surrounded by encapsulation layer20, and preferably has a width that is substantially equal to w. In someembodiments, each MTJ has a circular shape where both of the x-axis andy-axis dimensions are equal to w. In other embodiments (not shown), thetop-down shape may be elliptical or polygonal such that the x-axisdimension is unequal to the y-axis dimension w.

The present disclosure also encompasses embodiments where the MTJ stackof layers has a top spin valve configuration. According to theembodiment depicted in FIG. 13, a MTJ stack of layers 3 has an optionalseed layer 11, the doped Hk enhancing layer 17, free layer 14, tunnelbarrier 13, pinned layer 12, and cap layer 16 sequentially formed onsubstrate 10. The doped Hk enhancing layer is formed according to one ofthe aforementioned embodiments described with respect to FIGS. 6-9except for the underlying layer, which is a seed layer, rather than thefree layer in previous embodiments. Interface 40 is between the freelayer and tunnel barrier layer, and a second interface 41 is between thefree layer and Hk enhancing layer.

Referring to FIG. 14, the MTJ patterning sequence described earlier withrespect to FIGS. 10-11 is followed. Thereafter, encapsulation layer 20is deposited and planarized. Accordingly, sidewall 3 s is generated on aplurality of MTJ cells each having a surface 16 t that is coplanar withtop surface 20 t of the encapsulation layer.

Thereafter, a top electrode layer comprised of a plurality of topconductive lines (i.e. source lines) is formed on the MTJ array suchthat a top conductive line (not shown) contacts a top surface 16 t ofcap layer 16 in each MTJ cell. Thus, there may be a bit line below eachMTJ cell and a source line above each MTJ cell to enable read and writecurrents through the memory device.

All of the embodiments described herein may be incorporated in amanufacturing scheme with standard tools and processes. Moreover,throughput and cost of ownership (COO) remains essentially the same asin conventional memory fabrication schemes.

While the present disclosure has been particularly shown and describedwith reference to, the preferred embodiment thereof, it will beunderstood by those skilled in the art that various changes in form anddetails may be made without departing from the spirit and scope of thisdisclosure.

What is claimed is:
 1. A method comprising: forming a stack of magnetictunnel junction (MTJ) layers over a substrate that includes forming adoped metal oxide layer, wherein the doped metal oxide includes a dopantspecies selected from the group consisting of N, S, Se, P, C, Te, As,Sb, or Bi; patterning the stack of MTJ layers to form a patterned MTJstructure; and encapsulating the patterned MTJ structure.
 2. The methodof claim 1, wherein forming the doped metal oxide layer includesperforming a sputter deposition process that includes using a metaloxide target being sputtered by the selected dopant to form the dopedmetal oxide layer.
 3. The method of claim 1, wherein the doped metaloxide layer includes a metal species (M), an oxygen species (O) and thedopant species (D), and wherein forming the doped metal oxide layerincludes performing a sputter deposition process that includessputtering a target having the M, O and D.
 4. The method of claim 1,wherein the doped metal oxide layer includes a metal species (M), anoxygen species (O) and the dopant species (D), and wherein forming thedoped metal oxide layer includes: sputter depositing a target comprisedof M and O to form a metal oxide layer; and exposing the metal oxidelayer to the selected D in gas form.
 5. The method of claim 1, whereinthe doped metal oxide layer includes a metal species (M), an oxygenspecies (O) and the dopant species (D), and wherein forming the dopedmetal oxide layer includes: depositing a layer that includes M; andexposing the layer to a reactive gas environment containing O and theselected D in gas form.
 6. The method of claim 1, wherein the dopedmetal oxide layer includes a metal species (M), an oxygen species (O)and the dopant species (D), and wherein forming the doped metal oxidelayer includes: depositing a layer that includes M; oxidizing the layerwith O to a metal oxide layer; and exposing the metal oxide layer to areactive gas environment containing the selected D in gas form.
 7. Themethod of claim 1, wherein forming the doped metal oxide layer includes:forming a metal oxide layer over a substrate; and implanting theselected D into the metal oxide layer to form the doped metal oxidelayer.
 8. The method of claim 1, wherein forming the doped metal oxidelayer includes: forming a metal oxide layer over a substrate; andforming a doped layer that includes the selected dopant over the metaloxide layer; and performing an annealing process on the doped layer todrive the selected dopant into the metal oxide layer to thereby form thedoped metal oxide layer.
 9. A method comprising: forming a free layerover a substrate; forming a metal oxide layer over the free layer;converting the metal oxide layer into a doped metal oxide layer byincorporating a dopant selected from the group consisting of N, S, Se,P, C, Te, As, Sb, or Bi; forming a cap layer over the doped metal oxidelayer.
 10. The method of claim 9, wherein forming the metal oxide layerincludes sputter depositing a target comprised of a metal species andoxygen to form a metal oxide layer, and wherein converting the metaloxide layer into the doped metal oxide layer includes exposing the metaloxide layer to the selected dopant in gas form.
 11. The method of claim9, wherein forming the metal oxide layer includes depositing a metalcontaining layer and oxidizing the metal containing layer to form themetal oxide layer, and wherein converting the metal oxide layer into thedoped metal oxide layer includes exposing the metal oxide layer to areactive gas environment containing the selected dopant in gas form. 12.The method of claim 9, wherein converting the metal oxide layer into thedoped metal oxide layer includes implanting the selected dopant into themetal oxide layer to form the doped metal oxide layer.
 13. The method ofclaim 9, herein converting the metal oxide layer into the doped metaloxide layer includes: forming a doped layer that includes the selecteddopant over the metal oxide layer; and performing an annealing processon the doped layer to drive the selected dopant from the doped layerinto the metal oxide layer to thereby form the doped metal oxide layer.14. The method of claim 9, wherein the doped metal oxide layer has alower portion that is proximate the free layer and an upper portion thatis disposed over the lower portion, wherein a concentration of theselected dopant in the upper portion of the doped metal oxide layer isgreater than a concentration of the selected dopant in the lower portionof the doped metal oxide layer, wherein the dopant is present in thelower portion and the upper portion of the doped metal oxide layer 15.The method of claim 9, further comprising: patterning the cap layer, thedoped metal oxide layer and the free layer to form a patterned magnetictunnel junction (MTJ) structure; and encapsulating the patterned MTJstructure.
 16. A method comprising: forming a first layer over asubstrate; forming a metal layer over the free layer; converting themetal layer into a doped metal oxide layer, the doped metal oxide layerincluding a dopant selected from the group consisting of N, S, Se, P, C,Te, As, Sb, or Bi; forming a second layer over the doped metal oxidelayer.
 17. The method of claim 16, wherein converting the metal layerinto the doped metal oxide layer includes exposing the metal layer to areactive gas environment that includes oxygen and the selected dopant ingas form.
 18. The method of claim 16, wherein converting the metal layerinto the doped metal oxide layer includes: performing an oxidationprocess on the metal layer to form a metal oxide layer; and exposing themetal oxide layer to a reactive gas environment that includes theselected dopant in gas form
 19. The method of claim 16, whereinconverting the metal layer into the doped metal oxide layer includes:performing an oxidation process on the metal layer to form a metal oxidelayer; and forming a doped layer on the metal oxide layer, the dopedlayer including the selected dopant; patterning the first layer, themetal oxide layer and the second layer to form a patterned magnetictunnel junction (MTJ) structure; and performing an annealing process todrive the selected dopant from the doped layer into the metal oxidelayer to thereby form the doped metal oxide layer after patterning thefirst layer, the metal oxide layer and the second layer to form thepatterned magnetic tunnel junction (MTJ) structure.
 20. The method ofclaim 16, wherein the first layer is a seed layer and the second layeris a free layer.